Routing and Wavelength Assignment (RWA) is a well-known problem for fixed grid optical networks while Routing and Spectrum Assignment (RSA) is its equivalent term to the same problem for flexible grid optical networks. In fixed grid optical networks, wavelengths are spaced apart from each other according to a wavelength spectrum grid defined by International Telecommunication Union (ITU) in ITU-T G.694.1 (02/12), “Spectral grids for WDM applications: DWDM frequency grid,” the contents of which are incorporated by reference. In flexible grid optical networks, which is also described in ITU Recommendation G.694.1 “Spectral grids for WDM applications: DWDM frequency grid” (02/12), each signal can be allocated to spectrum with different widths optimized for the bandwidth requirements of the particular bit rate and modulation scheme of the individual channels. The ultimate objective of RWA or RSA is to find a wavelength or spectrum assignment on a route for a particular channel in the optical network, such assignment and routing being optimal in some manner.
A wide variety of static and dynamic RWA and RSA optimization algorithms are known. However, these approaches do not address partial reconfiguration of a set of services or do so with the pragmatic perspective of the operator (performance versus cost or performance versus service disruption/interruption tradeoffs), and furthermore, even where optimization is proposed, none of them attempt to minimize the real-world implementation cost, i.e., the number of steps involved in the reconfiguration. Note, a service can be a photonic Subnetwork Connection (p-SNC) which is managed by a control plane. Where partial re-configuration may have been considered, RWA and RSA algorithms are applied in consideration of all the p-SNCs in the network that can be reconfigured, exactly as it would be done for a greenfield deployment (new). Once optimization is obtained, reconfiguration of p-SNCs proceeds sequentially until the wavelength count reduction objective is attained, but without bound or even knowledge of the number of steps required or the temporary wavelength allocation requirements.